In general, a heat source device (for example, a centrifugal chiller and an absorption chiller) that supplies chilled water (heat medium) to a heat load of an air conditioner or the like is operated under rated conditions so as to cool chilled water at, for example, 12° C. up to 7° C. and supply the chilled water to the heat load. According to a known configuration for enabling the supply of chilled water having a temperature difference larger than such a rated temperature difference of 5° C., two heat source devices are connected in series to the chilled water (see, for example, PTL 1). According to this configuration, for example, the upstream heat source device cools the chilled water from 17° C. to 12° C., and the downstream heat source device cools the chilled water from 12° C. to 7° C., thus enabling an operation with a temperature difference of 10° C. from 17° C. to 7° C.